Heartworm Treatment Review

I've certainly talked a lot about heartworms in the past, and there is a ton of information out there on the web, and here, for that matter – my long seminar, problems with treatment, and more questions.

Still, a reader emails us with questions about the dog she adopted. 

The dog tested negative for heartworms at the shelter when acquired last year, but shows up positive (positive = bad) this year at check-up time.  She gave lots of background and had lots of questions, but it mostly boiled down to this one question:

"Any suggestions to maximize the chance we have a happy outcome?"  And here's my answer.

Under the circumstances you described, with a negative test last year, and a positive test this year, one would expect the dog to have only a small burden of adult heartworms.  This is not guaranteed, and would depend on the level of mosquito exposure and the dog's individual resistance to the parasites.

It is certainly important to continue giving your heartworm preventive medicine regularly.  

If you want to do "everything", then you could certainly ask for the doxycycline.  Doxycycline inhibits Wohlbachia, a microbe that is beneficial to the heartworm.

Giving this medicine for 4 weeks prior to the Immiticide injections (to kill the heartworms) causes the worms to become weaker (thus more susceptible to being killed by the medicine) and actually physically smaller (so less junk to clog up the blood vessels, and less for the white blood cells to have to clear out).

Treating the adult heartworms in two stages is considered to be safer than killing them all at once.  If you have only a few worms, this would not be a big benefit.  On the other hand, if you have a lot of worms, then killing half now and half later would be easier on the dog when the dead worms go downstream and lodge.

The two-stage treatment consists of giving only a single injection of Immiticide (which will kill the weaker worms), then four more weeks of doxycycline.  When the body has gotten rid of the first batch, one gives the two injections, 24 hours apart, and this kills the rest of the worms.

Two injections given 24 hours apart will kill all of the worms in most dogs, but not all.  This is true of the 3-injection, 2-stage protocol also.

Giving the corticosteroids (I use prednisone) immediately following the Immiticide is not a universal practice.  However, since I heard Dr. Tom Nelson (past president of the American Heartworm Society) recommend it, I have thought that it makes sense.  You know that inflammation will occur, so why wait until it gets bad enough to make the dog sick?  (Which is what I did "forever" until three years ago.)

Now we give a strong anti-inflammatory dose of prednisone daily for the first seven days, then half that once daily for 7 days, then continue that lower dose once every 48 hours for 14 days.  This has resulted in fewer night calls, because fewer dogs develop complications (that we can see).  In fact very few dogs at all have developed outward complications.

Again, that is not "standard" across the profession, but I really think it is helping our patients.

Unfortunately, despite our best efforts,  there are no guarantees.  No matter what we do, sometimes a dead worm will lodge in such a way that a large vessel ruptures and the bleeding in the lungs is fatal.  This is very rare, but it does happen.

Keeping the dogs vigorous physical activity at a minimum is an important part of the aftercare.  Changes in your dog's attitude can reflect a problem coming on, so let your veterinarian know sooner, rather than later.  Get the dog checked BEFORE it becomes an emergency.

Most complications are minor and treatable if they do occur. 

Six weeks after the final injection of Immiticide, we recheck the patient (if not before), and treat for microfilariae (the baby heartworms) if there were any present.  At this point, we should be "out of the woods".  It is very rare for complications to occur after this time.  Four months following treatment, a blood test is done to be sure that all the adult heartworms are gone.

Leaving the dog untreated is no solution, as the worms can cause a similar problem even if you have not yet treated the dog with Immiticide to kill them. 

We treated a dog this week who was known to be heartworm positive.  They had put postponed treating her.  Sunday evening she had severe pulmonary hemorrhage, and coughed up a LOT of blood.  She's on doxycycline and prednisone and cage rest now.  I hope we will be successful in getting her through the whole process, but we've got a long  haul ahead of us.

60 thoughts on “Heartworm Treatment Review

  1. Kellie Olinsky says:

    I have a 12-15 mouth old Bullmastiff that I just rescued with B2-Class 3 heartworm. I do trust my vet. But I was just confussed because I was told to give him interceptor 3 days after the first tx with immiticide it was one dose and then in 35 days the next 2 injections will be given, He is on prednisone.I am suppose to give the interceptor on Fri. 9-17-2010 Just scared because I have always been told that you do not give interceptor to dogs with heartworm.

  2. Doc says:

    Hello, Kellie,

    Sorry to be so late with a reply. I was out of town for several days.

    The first heartworm preventive medicine was diethylcarbamazine, also known as D.E.C. This preventive had to be given daily. If given to a dog with heartworms, there was about a one in three chance that the dog would die – Russian Roulette with two bullets.

    The monthly preventives have carried warnings not to give to dogs unless they are tested negative for heartworms. Now that these drugs have been around for quite a while, we find that the risk is not as high as previously thought.

    Selamectin (Revolution) is actually FDA approved as safe to be given to dogs that already have heartworms.

    The big boys in heartworm research tell us that ivermectin (Heartgard) is
    safe to use in heartworm-infected dogs, and actually recommend it when starting a dog’s treatment regimen. Several have told me that they don’t think that milbemycin (Interceptor) is as safe.

    That being said, I have had numerous patients who were taking Interceptor and got heartworms anyhow (missed dose or whatever) continue taking the medicine through their heartworm treatment with no trouble.

    It sounds like your veterinarian has had a similar experience, and isn’t worried about it. While the “authorities” recommend a different medicine, the risk with the Interceptor appears to be low. In my own practice, if the dog has not been on a heartworm preventive, we are starting them with Heartgard in cases like this.

    If you have misgivings, it is better to discuss them with your veterinarian directly.

    Good luck.

  3. Brittany Miser says:

    Hello, I’m in a bit of a pickle. My 9yr old pit bull lola-mae tested positive LAST year for adult heartworms. My vet at the time chose not to proceed with ANY treatment aside from her monthly heart guard and chose not to inform me at all, instead to inform his assistant. A full year later, my new vet strongly disagreed with his conclusion and quickly began treatment just as you have recommended. I was only informed after she reviewed our file this week. My question is how much more “risky” is it now, one full year after positively tested, and I’d like to note that she receives her monthly dose of heart guard all year and I’ve never missed a treatment. P.s. her proin treatment for urinary incontinence is now failing almost completely due to treatment…

    • Laura Jones says:

      My dog got his third injection/heartworm treatment 7 days ago. When should I start him back on heartworm Preventative? Was outside today (in North Carolina) and already saw mosquitos. I just adopted him and he was HW positive. Thanks

  4. Doc says:

    Hello, Brittany,

    Since you have been giving the heartgard regularly, there shouldn’t be any more heartworms than there were one year ago. Any increase in risk would probably be minimal.

    I assume the dog is taking some kind of corticosteroid like prednisone if you are having incontinence problems. That will get better when you get off the steroid.

  5. Suzy Mongeon says:

    In January I adopted a dog who had recently been treated for heartworm with 2 injections of Immiticide. According to the rescue group, he was asymptomatic and the worms were found during routine tests. I did some reading but apparently not enough, and didn’t realize the importance of keeping him quiet during the recovery period. He did go on Doxycycline when my vet noticed a cough reflex in his initial exam, right after he arrived from Tennessee. His vet down there had sent a dose of Ivermectin which I gave a month later. About 2 weeks ago he had a coughing fit which prompted another visit to the vet and a prescription for Baytril. When he failed to improve and actually seemed to go downhill in terms of appetite and energy we went back to the vet. His temperature has gone from 102.1 (previous visit) to 104.7 and his white blood cell count is low. Also he has some abdominal discomfort. He’s had x-rays – abdominal (showed a little buckshot!)and chest – bloodwork of course, now is on Cephalexin. We are treating symptoms of an upper respiratory infection, but I’m afraid the real problem is the dissolving worms and my ignorance! Lee is mixed breed and just under 20 pounds. He eats but not with his usual gusto, bowel movements seem okay, drinks water as usual. He just isn’t improving and the bills are piling up. I am determined to get him through this! Any advice? Would Prednisone be helpful at this point?

  6. Doc says:

    Hello, Suzy,

    The worms are usually pretty well gone by six weeks after the injections. It is certainly possible that the breakup of the worms caused some damage that has predisposed your dog to a respiratory infection. However, it is not likely that there are still worms present.

    I understand your concern. Sometimes a low dose of corticosteroid (like prednisone) for a short period of time helps with the inflammation in a respiratory infection.

    If antibiotics don’t seem to be helping, I do worry about a fungus infection like blastomycosis or histoplasmosis. Regular antibiotics do not work for these. Mira Vista laboratories can do a test for these using a urine specimen.

    IF fever is down and he is improving on the antibiotics, I would stay the course. It is unlikely that dissolving worms are still a factor.

    It sounds like your veterinarian is being very thorough.

  7. Sallie dim says:

    My dog just tested positive for heartworm along with arthritis and has tested positive for Lyme’s and antiplasmosis for years..we brought him in because of joint stiffness as he gets that each summer it seems from perhaps the Lyme’s and doxy clears it up but the vet insisted we come in and claims the two tests for h worm were positive and inactive microfilaria were seen in blood.. Started on doxy and gabapentin but worried about prednisone side effects and very worried about the h worm shots…is the prednisone for the arthritis or to help with the dying worms not hurting and clogging his organs and at age 11 do you feel the slow treat ongoing hrehreartguard is a more reasonable option? Of course after the appt and a couple doses of vetformin he’s running around looking fine but panting more so I am hopeful we’re not getting duped…

  8. Doc says:

    Hello, Sallie,
    Doxycycline is an antibiotic that is the drug of choice for tick-borne diseases, like Lyme. It also has some anti-inflammatory effects.

    Gabapentin is believed to be helpful with pain associated with nerve inflammation.

    I don’t know what vetformin is.

    Prednisone is a potent anti-inflammatory. It would certainly help relieve arthritis. With heartworm disease, we use it to reduce the swelling in the wall of the arteries where the worms are banging around. This keeps the arteries more open, and lets the blood flow more freely. There is less damage in the lungs, and it is easier for the heart to pump the blood through the lungs.

    Prednisone has other effects in the body. It can cause fluid retention in some patients. It can affect carbohydrate metabolism. Too much can suppress the body’s defenses. Long term excessive use causes even more problems.

    It can be very safe when dosed properly, and very beneficial. It depends on what is going on the the individual dog.

    It sounds like you have been working with this doctor for years. I doubt you are being duped. If in doubt, you could take the dog to another doctor for testing.

    The doxycycline can cause death of the baby heartworms in the blood, causing a temporary false negative test. However, the antigen test for adult heartworm protein in the blood would not be affected.

  9. Lorraine says:

    I rescued a chihuahua with Heartworm 2 months ago! She was treated in May with 2 shots of Immitcide. We brought her home a week later kept her low key! Took her to my vet cause she also had giardia and tape worm, treated for that also! Kept on pregnisone for cough she developed! 1/4 5 mg pill 10 days then 1/8 for ten days! She’s still coughing and it’s now just about 2 months later!
    I did wind up at the ER one night after taking interceptor, she was having trouble breathing! It’s time for her Heartworm med again and I’m worried about giving her the interceptor!

  10. Doc says:

    Hello, Lorraine,

    It is unusual to still have coughing at this time. You really should get her to your veterinarian for a recheck examination. It may be residual damage from the heartworm break-up (might need some more prednisone), or it could be something entirely different. Don’t just assume it is the heartworm treatment.

    Interceptor is usually very safe. It is not recommended to give if the dog has baby heartworms circulating in the blood.

    Of course, it is possible that your dog just doesn’t do well with the product. Some people cannot take an aspirin without getting sick. So, any medicine could be a problem.

    Talk with your veterinarian about this. If you decide together to go with Interceptor again, be sure to give early in the day so that any problems can be seen during regular hours and save that emergency fee.

  11. Melissa R Hulsey says:

    My vet did not use doxycycline prior to the two Immiticide shots back to back, but said he would be on it afterwards. Is this ok?

  12. Doc says:

    Hello, Melissa,
    My understanding is that the doxycycline prior to the Immiticide injections is used to inhibit the Wohlbachia organism that is beneficial to the heartworm. This makes the heartworms weaker and physically smaller, thus easier to kill and less for the body to dissolve afterward.
    Giving it afterward would have an anti-inflammatory effect, which could help the body deal with the breakup of the dead worms.
    I’m sure your doctor has a good reason for doing the treatment as he is. It is not possible for me to know your case like the doctor who is actually seeing your dog.

  13. Alison Howe says:

    I adopted my dog 2 years ago. She was heart worm negative for those 2 years and has been on Sentinel since being rescued. Her annual appointment she came up positive for heart worm. Her records showed her last vet had underdosed her. She was on up to 25 lbs and weight 27 lbs. We live in MA so heart worms are not very common. Now they want to start treatment. I’m so confused to how this could have happened and what to do? She tested negative for reproduction so it’s what’s in there now. I hate to put her through this.

  14. Doc says:

    Hello, Alison,

    I understand your reluctance. I can say that she probably has very few heartworms, so is unlikely to experience much in the way of side-effects when the worms die and shift position.

    When you say “negative for reproduction” I assume you mean that there were no microfilariae (microscopic baby heartworms) in the blood. The main importance of this would be dog being more likely to have a bad reaction when taking heartworm preventive if babies were present.

    The absence of babies in the blood doesn’t really tell you anything about the number of worms or severity of disease.

    In this case, you’ve been on preventive, but at a slightly low dose. This is why I suspect that you have very few worms present.

    You should also know that if there are fewer than four adult female worms present, the tests can give a false negative result. This means it is possible that your dog has had a very small number of worms all along, and that things are no worse than they have been.

    Again, I would not expect many problems with the treatment in your situation. As always, the doctor best equipped to advise you is the one actually seeing your dog.

    You should discuss your concerns with him/her.

  15. Colleen says:

    Hello, my dog Tinker (chihuahua) has been diagnosed with heart worm. The vet immediately put her on
    Doxycycline and Prednisolone with a monthly Dose of Tri-Heart Plus. Tinker has been taking as prescribed and with food for ease on the tummy. She seems to be having no issues with the meds, but I have noticed a little cough here and there. Is this normal? I have read that while the worms are dying a cough may develop, could this be the case?

    Thank you for your time!
    Extremely worried doggy Mommy

  16. Doc says:

    Hello, Colleen,

    It is very unlikely that there are any worms dying at this point.

    If the cough continues, you should let your veterinarian re-examine the dog.

  17. Tammy Rourke says:


    I recently adopted a Rottweiler X from the local shelter. During her stay at the shelter, she was treated for heartworm, with her last treatment on Oct. 22. 2017. I would like to know if it is safe to give the Interceptor Plus I received from her new Vet immediately or if I should wait until Nov. 22. I don’t want to leave her vulnerable or inadvertently make her sick. Also, I’m wondering if it is necessary for her to be on the “Plus” preventative, as I am concerned about possible side effects.

    Thanks for your attention to my query.

    Tammy and Gypsy

  18. Doc says:

    Hello, Tammy,

    The safety of the Interceptor Plus would depend on whether there are circulating microfilariae (microscopic baby heartworms) in the blood. If so, some dogs are more likely to have a bad reaction with milbemycin (the active ingredient) than they are with the low dose of ivermectin found in Heartgard. You should talk with your veterinarian about this.

    In Interceptor Plus, the plus refers to the addition of praziquantel, which kills tapeworms in the bowel. The milbemycin can kill hookworm, whipworm and roundworm in the bowel. The “Plus” part is super safe, not a problem.

    In Heartgard plus, the plus refers to pyrantel pamoate, which kills hookworm and roundworm in the bowel. Also super safe.

  19. Sally Martinez says:

    Hi. I have a 6 yr old GSD. Tested positive for heartworm. We started heartguard, doxy bid x 180 days and benadryl x 3 days. We are on day 6. Last night she started having incontinence issues. I’ve been putting the doxy in small PB sandwich or hot dogs. She won’t eat it in 1 bit anymore. She nibbles around and leaves the pills on the floor. Last night I had to squish the PB into a ball so she couldn’t pick it out. She eats about 1/4 what she use to. Guess my question is: can the doxy be causing the incontinence? I know dogs are smart… is she picking out the pills because its making her feel bad? 180 days is a long time…. I appreciate your advice. -Sally

  20. Doc says:

    Hello, Sally,
    My experience with doxycycline has been that side-effects are more commonly associated with irritation of the stomach, so poor appetite, vomiting, that sort of thing. I did a search on Veterinary Information Network and found more than one citation where specialists did not think that doxycycline would cause urinary issues.

    If she bites into the stuff, it tastes bad. Those capsules need to go down whole, and be followed by some food or water. Most dogs do better with the drug if they already have food on their stomach.

    You may already have some stomach ulceration, and if so, some time off the doxy and some stomach protectants would be indicated. You should discuss this with your veterinarian.

    I’m assuming that you are planning the so-called “slow kill” treatment with six months of doxycycline. Not a fan. In six months you could get the dog through the entire conventional treatment regimen, and you’d know the worms were killed, and you would know when they were dying, so you would know when to keep the dog quiet.

    Of course, I don’t know your dog’s situation, but you really should discuss your concerns with your veterinarian. The present situation is not okay.

  21. Andreea Smeu-Tanase says:

    Hi! We adopted a senior dog with a heartworm diagnosis (I haven’t been communicated which stage) about 3 months ago. He was administered both Doxycicline and Prednison, exactly in the scheme you mentioned, from the 10th of November until the 10th of December. Simultaneously he has been receiving daily treatment with Vetmedin and a human substitute for Cardalis, as Cardalis was way too expensive for our budget. On a monthly basis he receives a dose of Advocate. Since the 10th of December, he is only on Vetmedin, the mixture instead of Cardalis and Advocate and should receive his first shot of Immiticide soon. My questions/concerns I would highly apreciate help with:
    – I need to postpone this first shot for the next 3 weeks, due to some financial issues. As I understand, the prednisone and doxycicline treatment he received were aimed to make the adult worms smaller and weaker, as time goes by, will they grow back, get stronger and be more likely to cause problems? Can these 3 weeks make a real difference in the efficiency of the treatment and how well the dog feels afterwards? I emphasise the fact that he does not cough at all and in October, before the treatment, he did..
    – the dog is 12 years old, therefore not very active, but the “cage rest” expression makes me wonder. I cannot keep him locked, he will be staying with us in the house, where he has 3 stairs he needs to climb at the front door and 2 stairs in the room he usually sleeps in, when coming from the hallway – is this considered exercise and could affect him during treatment? He does not jump or run, but gets excited when we come home and sometimes growls at the cats, which can also mean increased blood flow, correct? Is this dangerous also? I don’t want to lose him due to side effects, as he is now in a good state, much better than 3 months ago. So I really don’t know what to do – the vet insists on doing the whole treatment (with one dose of Imitticide and a month later 2 doses – a day apart) but keeps on emphasising that, if we cannot provide “cage rest”, he is at risk.. Is this risk bigger than not administering the treatment at all?
    – does Immiticide interfere in any way with the Advocate administered?

    Thanks again for your advice!

  22. Doc says:

    Hello, Andreea,

    The Advocate will not interfere with the Immiticide (or vice versa).

    The three weeks will not make a big difference. The effect of the doxycycline is supposed to last about 3 months.

    Cage rest is perhaps an exaggeration. No running loose, no aggressive or active play. Walking around the house, even with a couple of steps should not be a problem.

    The veterinarian seeing your dog is the person best equipped to advise you. I can only give general information.

  23. Meagan says:

    Hi, I have an almost 11 month old German Shepard. He tested positive in February at 8 months old. He’s brother who was adopted at the same time also test positive. They had no symptoms of being heartworm positive.
    I feel like I wasn’t given much information from the vet about the whole heart worm treatment and options out there. We did the month of doxycycline and this past Thursday he got his first injection and we also started prednisone. He is doing ok, just low energy at times, panting and increased thirst and urination. It’s very hard to keep him calm and still.
    Is there anyway possible that with his young age that just staying on the heart guard chewables could have eventually made him heart guard negative?
    Also is there such a thing of only doing one injection and just staying on heart guard and that getting rid of the adult worms?
    I just don’t want to put him through this if I don’t have to.

  24. Deborah says:


    I adopted my dog in February 2017. He was HW-negative then, and negative in April 2017. He was on Heartguard from June-November 2017 (in Canada it’s only given for 6 months). Then he tested positive in April 2018. No microfillaria, my vet did an ultrasound of his heart and lungs and didn’t see any worms, so she thinks he has a very light case.
    He has been on Doxy for 1 month, and will probably start the melarsomine injections next week. Would it be ok for him to climb stairs after he starts the injections? I am worried about keeping him calm — he’s around 2.5-3 yrs old, and a very hyper and active husky/malamute mix. Anything gets him excited. A filled Kong also makes him hyper rather than keeping him calm… He will go in his crate without a problem provided the door is kept open. If we close the door he throws a little bit of a fit, and I’m concerned that even if it’s not a total fit it will still be too much excitement for him. So I am wondering how quiet I need to keep him if he has a small number of worms? Thank you!

  25. Doc says:

    Hello, Meagan,
    This got lost in the shuffle.

    The prednisone causes the increased urination, so then they do need to drink more water to make up for that.

    The single injection of melarsomine (Immiticide or Diroban) usually kills the weakest worms (youngest, oldest, males), but won’t kill the more vigorous worms. It takes the two injections, 24 hours apart, to complete the kill.

    If you just continue with the Heartgard (never missing a dose) the dog will probably eventually test negative after a few years. With a young, vigorous, active dog, a small number of worms may cause a little more damage to the arteries in the lungs than they would in a very calm, couch-potato type.

    If it were me, I’d go on and get the full treatment done.

  26. Doc says:

    Hello, Deborah,

    If your dog has that small a number of worms, it is very unlikely that he will have ill effects after the treatment. I wouldn’t worry about the stairs. Just don’t let him run loose, or encourage him to go nuts, chasing frisbees, etc.

  27. Deborah says:

    Thank you! He’ll be getting the injections next Monday. My vet wants to do the 2 injections 24 hrs apart rather than 1 injection, then 2 24 hrs apart 1 month later. Is this ok?? I thought that was the old protocol… I had her do a full blood panel and she said everything is pretty much normal, but his white and red blood cells count are a bit elevated.
    Thank you!!

  28. Doc says:

    Hello, Deborah,

    With a very small number of worms, using the two injections to kill them all at once shouldn’t increase your risk of bad reactions to the dead worms breaking up.

    There is some research that shows a higher percentage of kill when you do three rather than two injections. However, the same research also documented that some dogs never achieve 100% kill.

    I’d go with your veterinarian’s recommendations, as she is the person actually acquainted with your dog’s case.

  29. Claire Grady says:


    My 5-year old Great Pyrenees is currently taking doxy and will get her first injection next month for heartworms. However, the vet clinic said they would not be sending me home with prednisone. Is that normal? Seems like everyone else in this forum gets the shots and prednisone in conjunction.

  30. Doc says:

    Hello, Claire,

    Different doctors may have different protocols for treatment, based on their experience of what works best in their area.

    I have been happier since starting prednisone at time of treatment, rather than waiting to see if the dog develops complications.

    Prednisone is not an innocuous drug. It does have side-effects. You should discuss your concerns with your veterinarian.

  31. Florita says:

    My 1-1/2 year old lab mix tested positive and had the first shot 1-1/2 weeks ago and has been 4 days on the prednisone so far. I’ve been trying to keep him quiet but I’ve also been taking him leash walking, keeping him close to me. Should I not be taking him to walk at all? Each day he’s been a bit more lethargic than the day before and I don’t know if that’s because of the walks or the meds or both. Please advise. I’m very worried about him.

    • Doc says:

      Hello, Florita,
      If he is just walking calmly on the leash (versus struggling to drag you), there should be no problem with that.

      Most patients on prednisone feel good, but you occasionally get a dog (or person) that gets kind of moody and depressed. That will pass as the medicine is tapered off.

      He certainly could be feeling a little bad just because of the worms dying and moving.

      If the walks aren’t strenuous, they shouldn’t be a problem, and should make it easier for him to be calm the rest of the time.

  32. Caroline says:

    My 2 y/o pit/boxer/lab mix was diagnosed with heartworm in august 2021, treated with doxy that same month, and just got her first injection last week (delay in treatment due to adoption and differing treatment opinions from vets). When she tested positive, it was a weak-positive result, and she had just recently tested negative. She has also never had symptoms. She is having such a hard time staying calm as she is a very energetic dog. We are giving her sedating meds which help a lot but she seems depressed and anxious that she is not allowed to exercise and play anymore.

    Are dogs ever treated with just one injection if it is a new infection or is it recommended to always to the 3 total injections? I want to do what is best for her but I don’t want her to be this miserable for 3 months.

    • Doc says:

      Hello, Caroline,
      We sometimes treat with just the two injections, 24 hours apart, in a situation like this, but a single injection is very unlikely to be adequate.

      It’s usually okay to go for walks on a leash, and dogs inside the house are not going to be doing prolonged aerobic exercise, so normal activity in the home is usually okay.

  33. Debbie says:

    Hello! We adopted our Miniature Bull Terrier “Gypsy” from Breed Rescue May 27, 2021. She was HW+ when the Rescue took her in from a Texas Shelter. The Rescue did 3 injections of Immiticide according to AHS protocol & followed guidelines for treatment to the letter! Her last injection was April 8, 2021. 10 mos. & 1 day (Feb 9, 2022) post 3rd injection we had her HW tested…result was a “faint” positive. 1 wk later she was retested by a reference Lab (Antech)& result was “borderline” positive. We opted to give 30 days of Doxy, wait 30 days & retest April 20th…we are praying for a Neg. If still positive the Plan is to give 2 injections back to back. The Rescue stated that she had a light load & was asymptotic prior to her treatment under her care…& she still remains asymptomatic @ this time. ?…the Rescue only gave her Deramaxx after ea injection & reportedly did very well w no issues…her current Vet used Pred aft injections but I would like to avoid its use if possible bc of the side effects. She is a high energy dog & I can’t imagine her on Pred & trying to keep her safe & calm during recovery while on Pred. What are your thoughts regarding the use of a Med like Deramaxx use after treatment versus Pred. We don’t know why she was a slight Pos aft 3 treatments unless the Strain of HW from Texas is resistant OR she has some “old” Antigen from previous infection still floating around & hopefully waiting 2/3 mos. will produce a Neg result. She was MF Neg on both tests as well. Please give me some advice & guidance so I can help my beloved girl! Thank you in advance!

    • Doc says:

      Hello, Debbie,
      So sorry that I’ve gotten behind. Deramaxx can help with pain and inflammation at the injection site, but is less likely to help with pulmonary artery inflammation. With a dog who has had the three-dose protocol and still showing a positive antigen test, my personal preference would be to just stay on preventive, or look at switching to Advantage Multi every month after the 4 weeks of doxycycline. Some dogs will never show 100% clear of the heartworm antigen protein, but they have no clinical disease. If chest films are okay, I’d be okay with skipping the second go-round of Immiticide. If she has very few heartworms, she is unlikely to have any difficulty with it, but it may not give a huge benefit, either. You have to realize my advice may not be as good as your veterinarian’s advice. They have seen the dog and I haven’t.

  34. Hannah says:

    Hello! I would greatly appreciate your opinion. I have a 14 month old labradoodle who I got at 8 weeks old. He’s been on trifexis since he was two months old (that’s when the vet advised to start based off his weight/age). I was very shocked to learn that he is strong heartworm positive whenever I took him for his vaccinations. He never had a heartworm test as a puppy (as it wasn’t recommended to me). I’m so upset and frustrated that my pup has heartworm after never missing a dose and being on preventative his whole life. My vet is advising 90 days of doxy and after 30 days of the doxy, two treatments 24 hours apart. Is this a normal recommendation? I’m freaking out about him and want to make sure I am going the best route for my pup. Thanks!

    • Doug says:

      I picked up my pit lab at 5 months old (oct 2021) no test was given until she was 11 months (April 2022). She tested positive and was put on doxy and prednisone for 30 days. She did take heart worm prevention pill in the first 30 days then day 60. Day 61 shot 1 then 24 hours later shot 2. Then put on prednisone for 30 days. We started everything April 25. It’s been a month since her second shot and will take her last prednisone pill July 26. I’ve limited physical exercise but no cage restrictions, just kept her calm. Last few days I’ve done longer walks and have her chasing her favorite toy for a minute or two. She seems so be doing very well. She will be retested in October- finger crossed she’ll be negative.

    • Doc says:

      Hello, Hannah, sorry I got so behind on my replies.
      There would be no reason to test an 8 weeks old puppy for heartworms. It takes six months after mosquito exposure and infection for the heartworms to mature to the point that they can be detected by testing.

      Trifexis does need to be given with a meal for best absorption, and you have to increase the dose as the puppy grows. If this was done, the manufacturer’s guarantee should cover the cost of treatment. There also should be VERY few worms present.

      I don’t know about the 90 days of doxycycline, we usually give 30 days. Then there’s a 30-day wait, and a single injection of melarsomine (Immiticide or Diroban), followed by the two injections 30 days later. With a very small number of worms (as we would expect here), I would be okay with skipping the single injection “pre-treatment”.

      I think you are almost certain to have a good outcome.

      If you treated appropriately with Trifexis and it wasn’t 100% effective, I would consider switching to Advantage Multi.

  35. Wendy says:

    I have a 13 year old dog who posted positive for heartworm and now is having heart murmur.

    The vet suggested giving heartgard, fortekor and also furosemide (since she retains fluid) for the first three days.

    My situation now is that I only can give her medicine once a day as I am only around her in the evening time.

    The vet accomodates and said to give her 3 tabs of doxy and 3 tabs of predisone but I feel like that is too much meds for her body to take.

    I am thinking of taking it down to 1 tabs per day for both and have her had the medicine until it finishes.

    Can I do that or that is not enough to kill the heartworm ?

    I dont feel like giving her predisone but I think it is necessary for heartworm treatment ?

    • Doc says:

      Hello, Wendy,

      Heartgard is to keep your dog from getting more heartworms in the future from ongoing mosquito exposure.

      Fortekor (benazepril) can reduce the resistance to flow that makes the heart’s work more difficult.

      Furosemide is a diuretic. It tells the kidneys not to save as much water. The kidneys initially filter the blood taking a little bit of waste and a lot of water. They then recirculated the filtrate to concentrate the urine, conserving water. The diuretic backs off that saving of water.

      In talking about drug dosages, you are not giving me the dog’s weight or the strength of the drugs. I have no idea if dosage is safe or effective (just aside from the fact that I have not examined your dog – your doctor is a better judge of that than I am).

      The prednisone is anti-inflammatory. The worms banging around in the pulmonary arteries cause inflammation. The arterial wall swells, thickens, which narrows the interior diameter of the blood vessel. This increases the workload of the heart, and the pressure in the artery. That can lead to fluid seepage from the artery into the lung tissue (that should be full of air instead of fluid), and even to broken arteries and bleeding into the lungs. The prednisone can relax and open those vessels to help avoid that.

      For the doxycycline to be most effective, it is given twice daily, as the drug doesn’t maintain a steady blood level if only given once daily. Standard recommendation has been 10mg/kg twice daily, but some studies have found equal effectiveness at 5mg/kg twice daily. Dogs are less likely to have stomach upset at the lower dose.

  36. Oscar says:

    What is the appropriate prednisone dosis? My 13 y.o lab had her three injections. I understand what prednisone does, but confused by the dosis schedule. After the first injection she was on it every 12hrs for the first week, every 24hr for the the second and then every 48 hrs. After the second and third injection of the vet re-started the schedule, but I didn’t hear a lot of confidence from the vet tech who gave me the instructions. Does this sound right? And if it’s, for how long after the 3rd injection should she be taking it?

    • Doc says:

      Hello, Oscar,

      While I usually go with once daily dosing, I start with the 1/2 mg per pound daily for 7 days, then 1/4 mg per pound for 7 days, then 1/4 mg per pound every 48 hours for 14 days. SO, if your doctor was using the same size tablet every dose, that would work out the same: you’re getting two tablets the first week, then one tablet the second week, then one tablet every 48 hours after that.

      The first week is really the time I am most worried about the dog needing the prednisone. After taking that big dose for a week, I want to taper it off gradually, so I usually wind up with four weeks total.

      There isn’t a hard and fast rule on that. It sounds to me like your doctor is following a reasonable protocol.

  37. Deb says:

    Hi. I’ve been reading all your messages. I have a 2 year old dog that just finished her injections almost a week ago. She seems to not do well on prednisone as she becomes manic. This happened for a week with her first injection until we started to taper it. But tonight she is panting along with being manic and she didn’t do that the last time. I’m worried that a worm could have logged in her lungs or something. Could this be possible after the final injections? We haven’t been walking her much and she’s had no activity off leash, but out in the yard last night she saw a fox abd started barking and I had to pull her away as she wanted to go towards it. Could this have caused complications? Also have you ever heard of this mania on prednisone? She constantly paces at night for hours. Any help is greatly appreciated! Thank you.

    • Doc says:

      Hello, Deb,
      While most dogs just “feel better” when taking prednisone, there are certainly a small number of dogs where it is mood-altering. Some get just sort of depressed, and rarely you have the kid that bounces off the walls. That is really rare, but if it’s your dog, it’s not much consolation to know that nobody else is having the problem. We usually start tapering the prednisone dose after a week, so that should hopefully make things a little better.

      About 4 to 7 days after the last melarsomine injections (Immiticide, Diroban) is when the worms die and shift position, so the timing is about right on having a little respiratory distress. If you keep her as quiet as possible and stay on your prednisone, this will probably be okay. If she gets any worse, you should contact your veterinarian. I don’t think that barking at the fox is likely to have made any significant difference.

  38. Chuck says:

    I just wanted to to thank you for all your kind observations and advice. I have a positive 4 yr old in Texas and have begun Doxy with injection 1 the first part of March. Hoping for the best as he has been on Heartgard since adoption one year ago with a negative test at that time. Your column is great. As you know sometimes listening helps the parents more than anything, and your light hand with wise, experiential advice is appreciated!

    • Doc says:

      Thanks very much for your kind words. Best of luck with the treatment. If he had a negative test last year, you probably have a low worm-burden and the prognosis is good.

    • Michele Duthrie says:

      I agree this has been so helpful, my dog just had his last shot so there are so many concerns and I have found this to be so helpful Thank you for taking the time to answer all these questions and concerns.

  39. Ali says:

    Hi Doc,

    We recently adopted a 2 year old dog. She tested negative for heartworm and began preventative medication with her foster family in November 2022, and then tested positive in March 2023 a few weeks after coming to us. She doesn’t seem to have any symptoms and has now been on doxy for 2 weeks.

    My question is about the benefit of taking the “month off” between finishing doxy and starting the melarsomine per the AHS rececommendations. How important is this? Because we’re still getting to know her and she’s still adjusting to our busy urban neighborhood, even the modest exercise restrictions we were recommended to start now is having an impact on training and bonding, so we’d like to get through the strict exercise restrictions as soon as possible!

    I can’t find any evidence for improved outcomes with that 30-day break between doxy and melarsomine. Thanks in advance for any insight you could offer, and thanks so much for all your help to everyone on this site!

    • Doc says:

      Hello, Ali,

      Taking the “month off” has been the standard recommendation. The rationale was that when you give the doxycycline to knock out the heartworm’s beneficial microorganism Wohlbachia that the worms would not only be weaker and easier to kill, and maybe less antigenic when they hit the smaller arteries downstream, but that they would also be somewhat smaller in size, so less blockage of the arteries.

      There are doctors who question the benefits of waiting the additional four weeks.

      I can’t find any clear data that prove it one way or the other.

      As far as restricting the dog’s activity, I don’t usually bother with that until they have received the melarsomine.

      That being said, there are those who recommend it as soon as the dog is diagnosed. There are certainly dogs who have worm migration that is unrelated to a heartworm treatment. These dogs can get pretty sick, have lung hemorrhages.

      In my experience, those cases are rare. In dogs that appear to be in good health, this rarely happens. If the dog already has clinical signs of poor exercise tolerance, coughing after exercise, weight loss, etcetera, then that is a different story.

      I practice in a severely endemic area for heartworms, lots of mosquitoes for most of the year. Acute cases of spontaneous hemorrhages in untreated dogs with small worm burdens are rare. I have seen maybe a half dozen in 44 years. It happens.

      My idea of exercise restriction in an otherwise healthy, asymptomatic dog is being free inside the home, and on a leash outside. No running loose.

      Having said all that, I have never examined your dog, and do not know your circumstances. I can only speak from my general experience. The doctor seeing your dog is your best source of information.

  40. Lori G says:

    My 5 year old Mini Aussie has been diagnosed with heartworm with no microfilaria present. She is 2 weeks into doxy and is due for for her 1st injection Memorial Day weekend after taking a month off. Due to the holiday, the vet wants to push the shot into the following week. Is it fine to wait the 4.5 weeks after finished doxy or should we have pulled it forward a week?

    • Doc says:

      Hello, Lori,
      It is absolutely fine to wait a few extra days. I am told that the beneficial effects of the doxycycline last 3 months or more.

  41. Ginger G says:

    Hello first of all I strongly agree that by listening/reading here has helped so much I have a 4years young chi/mix she means the world to me I was terably wrong to have put off giving her monthly preventative thought the weather was cold enough for a break she is now heartworm positive we are going to a recommended vet that is a low cost outfit my question is since the treatment plan is nowhere to be found and believe me I have had to have read everything regarding on this world wide web for a 2 injections of Diroban 24 hours apart stay 2 nights without me and then be sent home with Doxycycline and prednisone she is and have been on the Revolution topical for the 4 years we have her iwas given Sentinel Spectrum chewable 2weeks ago to give her I was told it was fine just give it to her even though I already gave her the Revolution but just before I was going to give it to her I happened to read per MFG the warning regarding giving to a HW positive dog was not at all a good idea so I just did not give it to her I know it could have helped to kill the babies and make this injection time now go smoother for her and the fact that they didn’t give the doxcycline instead am I doing the right thing I don’t want my little Hailey girl to die over too many killed at once all I can do is cry sorry for carring on and not just getting to the point I hope you understand my cituation this will only be the second time to this vet and they really just do Dental work no sick dogs but alot of rescue dogs please advise if you would thank you so much

  42. Ginger G says:

    Hi hope I can get to the point fast here my 4 yr chi/ mix positive HW test, her name is Hailey small with thin hair coat.my question is the low cost vet we are going to take her to has a different type of protocol the treatment consists of just 2 inj. 24 hours apart spend 2 nights there pick her up on the 3rd day be sent home with Doxycylone and Prednisolone and then retest 6months later …so nothing before hand am I doing the right thing? I have tried to ask questions but the answer is the same we don’t have to give the shots you can do the slow kill if you want. .I say no the worms will just get bigger ect.. I would like to know why it is so hard it seems for me anyways to talk to veterinarian s about the procedures at times and if I am doing the right thing for my Hailey girl please advise Thank you so much

    • Doc says:

      Hello, Ginger,
      Giving the two doses of Diroban may be just fine. The reason that we give a single dose, then wait a month for the two doses, is that we know the smaller, weaker worms will die with just a single injection. The idea is to kill some of the worms now, let the dog recover from this, then kill the rest of them a month later. That way you don’t have a large number of dead worms to deal with all at once. In this case, if you were giving preventive medication most of the time, just missed a dose, you probably have very few worms in the first place. There may not be a big advantage to doing the half treatment a month before the full treatment. If there are very few worms, giving the 2-dose treatment of Diroban wouldn’t really increase your risks of difficulty with the treatment. Sending home doxycycline AFTER the treatment is not what is usually done. You won’t be eliminating the Wohlbachia ahead of time, since you are killing the worms with the Diroban before you start it. Doxycycline does have some anti-inflammatory properties, so it may still help with the inflammation that results when the dead worms move downstream. That’s why we give the prednisone or prednisolone.

      Revolution is safe to give dogs even when they already have heartworms. The milbemycin oxime in the Sentinel Spectrum might cause a problem if the dog has microfilariae (microscopic baby heartworms) in the blood, but the likelihood is very small. We have given it with no problems. American Heartworm Society recommends staying on heartworm preventive during treatment, but prefers Heartgard, Revolution or Advantage over Sentinel or Interceptor.

      If she has microfilariae in her bloodstream, then we usually do give medication to treat for those about six weeks after the Diroban.

      • Ginger G says:

        Hi Doc.I wanted to get back to you with an update on my little dog Hailey. May 1st was the date for Hailey to be given to the treating vets for her Diroban injections.She was going to be given one shot on May 1st Monday stay the night and get another shot on Tuesday May 2nd and come back home on Wednesday May 3rd. At that time she would be sent home as far as I know with the Doxycycline and the Prednisolone.
        So okay on May 1st I took Hailey in the treating vet did a 478 Heartworm Micofilaira Direct Drop Test 6-8 per field – positive
        DLAB039 Heartworm Test Results +
        HWSNAP Heart Worm Only Test
        When he came back out to return Hailey to me he said she was full of microfilaira and he postponed the injections. He sent me home with instructions to break up one Sentenal Spectrum chewable tab into 4 quarters given 3 days apart and wait 3weeks to check her again.
        My question is ,by giving her the Sentenal Spectrum would this be the same thing like giving the Doxycycline?
        He wanted me to give her the whole tab at once but I was reluctant to do so do to the warnings about too many worms dying too fast..
        So by giving her the quarters what will that have done to to the worms ?
        And FYI Hailey did fine after taking each quarter I work Midnight shift UPS and stayed home with her to make sure she would be alright. Hailey is done with the Sentenal Spectrum now she coughs and there is a slight gag as she trys to spot up but other than that seems to be herself. Thank you again for your time with me for my concerns it really does put my mind at ease and I have learned to stay positive with a smile on my face in front of Hailey to keep her from stressing out. I have talked with her about her situation and call me crazy but I think she understands somewhat 🙂
        PS. All the technical info regarding the Test is just what is on the bottom of my instruction sheet I was sent home with I really do not know what it all means could you possibly clairify?
        Thank you Ginger

        • Doc says:

          Hello, Ginger,

          The snap test is for an adult heartworm protein from the female heartworm reproductive tract. Positive means that the protein is present in the blood, meaning that you have adult heartworms present. Microfilariae are the microscopic baby heartworms. A positive direct test means that you see them looking directly at a drop of blood under the microscope without having to use a larger sample and concentrating it to detect them.

          The microfilariae are not affected by the Diroban. The milbemycin oxime in Sentinel Spectrum is supposed to kill microfilariae that have been injected by the mosquito in the past month. In some dogs, it can affect the circulating microfilariae in the blood (a much different life-stage of the parasite). For many years, it was recommended not to give it to heartworm-positive dogs, but there seems to be little evidence that it is more likely to cause a problem than giving Heartgard (low dose ivermectin), which is the product recommended by the American Heartworm Society for microfilaria positive dogs. Advantage Multi (moxidectin applied topically and absorbed through the skin) is also considered safe, as is Revolution (selamectin applied topically, but goes systemic, absorbed through the skin). I wouldn’t have a problem with giving the Sentinel Spectrum. You don’t want to acquire more heartworms through new mosquito exposure, so you need to be on some kind of preventive medicine.

          Dividing the dose of Sentinal Spectrum and giving it over several days probably makes it completely ineffective. I would be very suprised if it cleared any microfilariae from the blood. It has no effect on the adult heartworms at any rate.

          I am uncertain as to why your veterinarian has elected to modify the treatment protocol based on numbers of microfilariae in the blood. The number of microfilariae in the blood doesn’t have a direct relationship to the number of heartworms in the chest. Many dogs with a LOT of adult heartworms have no microfilariae at all. Lots of babies does not necessarily indicate lots of mommies, either.

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